If I connect a M2 Pro/Mac MacBook Pro 16" to a display of 5120x1440 or 3440x1440, it should work but at 60Hz. Am I correct?
Correct.
How come more than 60Hz at above 4K is not available?
If you're using 5K, 6K, 8K displays at 120hz or above, that's
a lot of data, especially if you're also using a wide colour space and dynamic range. So it comes with a lot of overheads and cost in terms of processing and transmitting this data. (Somewhat infamously, when the 2014 iMac came out, 5K even at 60Hz wasn't supported by the DisplayPort standard at the time, so Apple created a weird custom output with two separate DisplayPort streams which were then merged back together).
Relatedly, most 5K/6K monitors (not that there are many) are limited to 60Hz - e.g. the LG UltraFine, the Studio Display, and the Pro Display XDR are all 60Hz.
I'm not really sure what the use case would be anyway to be honest. Things like the Studio Display are intended for design work, where high pixel counts and colour accuracy matter, not refresh rates. Many gamers prioritise high/variable refresh rates prefer to play at 1440 or even 1080, not 4k. 5K and 6K aren't even in the picture.
It's always fun to have a higher number, and to fantasise about 8K monitors running at 480Hz, but at some point you're creating huge technical challenges without actually making something more useful.
Will M3 Pro and Max be the same?
Maybe! Or maybe they will be different!
Friend, you have posted more than 7,000 times in this forum - I would think that means you've been around long enough to know that Apple doesn't reveal specs ahead of time, and everything else is just speculation!